1. Field of the Invention
The invention relates to the processes for the manufacture of prosthetic elements, such as dental and maxillo-facial elements, and elements produced by such processes.
2. Discussion of Background Information
A number of materials were tested for the purpose of reconstructing or repairing hard biological tissues, namely bones or teeth. These materials, often called biomaterials, should have appropriate mechanical properties and not cause chemical reactions detrimental for tissues coming into contact with a prosthesis made with these materials.
In this respect, the qualities of metal oxides, in particular those of alumina, Al.sub.2 O.sub.3, the principal oxide of aluminum are known. Indeed, alumina is an extremely chemically stable compound, and is therefore considered as being inert in a biological medium. In addition, the hardness of alumina, four to five times higher, depending on its state, than the hardness of titanium alloys makes it a biomaterial which is mechanically superior to widely used titanium alloys. Moreover, the cost of titanium alloys is high compared with that of alumina. Therefore, alumina is particularly advantageous for the production of prostheses for use in a biological medium.
Known processes for manufacturing alumina prostheses, or prostheses coated with alumina, do however have serious disadvantages. Indeed, at present, it is now known how to use alumina except in the form of deposited or projected ceramics, i.e., the production of a layer of alumina either consists in the sintering of powders which are then amalgamated or in a thermal spraying process (by means of an electron torch, for example). These techniques, applied to the coating of prostheses or implants are described for example in the U.S. Pat. No. 4,556,389, in EP-211 676, in FR-2 318 617, and in International Application WO 87/00030.
However, these techniques are disadvantageous because:
either the layer of alumina deposited on the prosthesis is thin, and the porosity of the ceramic causes a risk of bacterial invasion which could lead to an infection,
or the layer is thick, around several tenths to several millimeters, and is fragile, and brittle, in spite of a porosity liable to promote the osteogenesis (regeneration of the bone around the prosthesis) without risk of infection.
Moreover, in both cases, the interface between the prosthesis and the layer of sintered or sprayed alumina is not unalterable since there only exists a mechanical retention between these two media. The layer of alumina is therefore not particularly adhesive.
Finally, should be noted that sintering and thermal spraying are not adapted to the deposit of ceramics on complex shapes, such as a threading, that occur frequently on prostheses or implanted prosthetic elements. These processes result in heterogeneous deposits of varying thickness covering the shapes. Moreover, even if it remains possible to reshape a profile in a layer of alumina ceramic which is sufficiently thick, the hardness of this oxide makes the work difficult and not very accurate.